linux-kernel - Re: [PATCH v2 05/11] KVM: x86: emulator: update the emulation mode after CR0 write

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Date:   Wed, 20 Jul 2022 23:50:03 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Maxim Levitsky <mlevitsk@...hat.com>
Cc:     kvm@...r.kernel.org, x86@...nel.org,
        Kees Cook <keescook@...omium.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        linux-kernel@...r.kernel.org, "H. Peter Anvin" <hpa@...or.com>,
        Borislav Petkov <bp@...en8.de>, Joerg Roedel <joro@...tes.org>,
        Ingo Molnar <mingo@...hat.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>
Subject: Re: [PATCH v2 05/11] KVM: x86: emulator: update the emulation mode
 after CR0 write

On Tue, Jun 21, 2022, Maxim Levitsky wrote:
> CR0.PE toggles real/protected mode, thus its update
> should update the emulation mode.
> 
> This is likely a benign bug because there is no writeback
> of state, other than the RIP increment, and when toggling
> CR0.PE, the CPU has to execute code from a very low memory address.
> 
> Signed-off-by: Maxim Levitsky <mlevitsk@...hat.com>
> ---
>  arch/x86/kvm/emulate.c | 13 ++++++++++++-
>  1 file changed, 12 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
> index 6f4632babc4cd8..002687d17f9364 100644
> --- a/arch/x86/kvm/emulate.c
> +++ b/arch/x86/kvm/emulate.c
> @@ -3659,11 +3659,22 @@ static int em_movbe(struct x86_emulate_ctxt *ctxt)
>  
>  static int em_cr_write(struct x86_emulate_ctxt *ctxt)
>  {
> -	if (ctxt->ops->set_cr(ctxt, ctxt->modrm_reg, ctxt->src.val))
> +	int cr_num = ctxt->modrm_reg;
> +	int r;
> +
> +	if (ctxt->ops->set_cr(ctxt, cr_num, ctxt->src.val))
>  		return emulate_gp(ctxt, 0);
>  
>  	/* Disable writeback. */
>  	ctxt->dst.type = OP_NONE;
> +
> +	if (cr_num == 0) {
> +		/* CR0 write might have updated CR0.PE */

Or toggled CR0.PG.  It's probably also worth noting that ->set_cr() handles side
effects to other registers, e.g. the lack of an EFER.LMA update makes this look
suspicious at first glance.

> +		r = update_emulation_mode(ctxt);
> +		if (r != X86EMUL_CONTINUE)
> +			return r;
> +	}
> +
>  	return X86EMUL_CONTINUE;
>  }
>  
> -- 
> 2.26.3
>